The present invention relates generally to communication systems, and more particularly to channel coding techniques for use in communication systems which incorporate multi-mode source encoders.
A multi-mode source encoder supports multiple coding modes, with each coding mode designed to provide optimal coding for a particular type of source signal. An example of a multi-mode source encoder known in the art is the multi-mode transform predictive coding (MTPC) encoder. The MTPC encoder is a wideband (7 kHz) encoder used for compression of speech and audio signals, at nominal source bit rates from about 13 kbits/sec to 40 kbits/sec. The MTPC encoder is based on a transform predictive coding paradigm which combines linear predictive coding with transform coding. This combination allows the MTPC encoder to incorporate both speech and audio coding principles into a single coding structure having a speech coding mode and an audio coding mode. Additional details regarding the MTPC encoder can be found in, e.g., S. A. Ramprashad, xe2x80x9cA multimode transform predictive coder (MTPC) for speech and audio,xe2x80x9d IEEE Workshop on Speech Coding, pp. 10-12, Porvoo, Finland, June 1999, and S. A. Ramprashad, xe2x80x9cHigh quality wideband embedded coding using an inherently layered coding paradigm,xe2x80x9d IEEE International Conference on Acoustics, Speech and Signal Processing, pp. II-1145 to II-1148, Istanbul, Turkey, June 2000, both of which are incorporated by reference herein.
In communication system applications, a compressed source-coded bit stream such as that generated by the MTPC is transmitted via a system transmission channel to a receiver that includes a source decoder. Examples of such transmission channels include wireless network channels such as cellular system channels and terrestrial and satellite digital broadcast channels, packet network channels such as asynchronous transfer mode (ATM) or Internet protocol (IP) channels, and circuit switched network channels such as integrated services digital network (ISDN) channels. During transmission, errors can be introduced into the source-coded bit stream. Channel coding techniques are often used to limit the impact of such transmission errors on reconstructed signal quality at the receiver.
An example channel coding technique utilized in IS-95 code division multiple access (CDMA) cellular systems is described in E. Cohen and H. -L. Lou, xe2x80x9cMulti-rate detection for the IS-95 forward traffic channels,xe2x80x9d Proceedings of the IEEE Global Telecommunications Conference (GLOBECOM), Vol. 3, pp. 1789-1793, Singapore, November 1995, which is incorporated by reference herein. In this technique, a source encoder can operate at a number of different rates, and the channel encoder configures the channel-coded frames so as to have the same size regardless of the particular source coding rate. For example, information source coded at xc2xd of a full rate is repeated twice within a given channel-coded frame such that the resulting channel-coded frame is the same size as that generated from information source coded at the full rate. Rate detection at the receiver is used to determine the particular channel coding configuration for a given frame.
Although these and many other channel coding techniques are known in the art, a need exists nonetheless for improved channel coding techniques particularly well suited for use with multi-mode source encoders such as the above-described MTPC encoder.
The present invention provides methods and apparatus for channel coding in a communication system that includes a multi-mode source encoder.
In accordance with one aspect of the invention, source-coded information from the multi-mode source encoder is channel coded for transmission in the communication system. A multi-mode channel encoder associates different channel coding error protection profiles with each of the modes of the multi-mode source encoder. The channel encoder determines a mode used by the multi-mode source encoder to source code a given frame or other designated portion of the information, and channel codes the source-coded portion of the information utilizing an error protection profile identified at least in part based on the determined mode.
An illustrative embodiment of the invention utilizes a cyclic redundancy check (CRC) or other error detecting code as an outer channel code, and a set of rate-compatible punctured convolutional (RCPC) codes as an inner channel code. The CRC in this illustrative embodiment protects a portion of each frame of a source-coded bit stream, and this CRC-protected portion for a given frame includes one or more mode bits identifying a source coding mode used to source code the given frame. The CRC may also be configured so as to be suitable for use in generating an error flag for triggering an error mitigation algorithm in a source decoder.
In accordance with another aspect of the invention, a multi-mode channel decoder is operative to hypothesize that a particular one of the modes of the multi-mode source encoder was used to source code the designated portion of the information. The channel decoder analyzes at least part of a set of corresponding channel-coded information using an error protection profile associated with the hypothesized mode of the multi-mode source encoder, in order to determine if the error protection profile and the hypothesized mode are appropriate for respective channel decoding and source decoding of the designated portion of the information. The part of the corresponding channel-coded information that is analyzed preferably is a part containing source mode bits generated for a given source-coded frame of the information by the multi-mode source encoder.
The particular one of the modes of the multi-mode source encoder hypothesized as having been used to source code the designated portion of the information may be selected by performing a Viterbi decoding each of the different modes, and selecting the hypothesized mode based at least in part on a result of the Viterbi decoding, e.g., based on which of the modes resulted in the best path metric. As another example, the particular one of the modes of the multi-mode source encoder hypothesized as having been used to source code the designated portion of the information may be selected based in part on mode bits from a previous successfully-decoded frame.
Advantageously, the present invention, by adapting the error protection profile based on the source coding mode, can provide improved reconstructed signal quality for a given channel signal-to-noise ratio (SNR), regardless of the particular source coding mode used and thus regardless of the type of input signal, e.g., speech or audio.